I have to completely agree that the constraints are what make these games great. One constraint not talked about thus far are the physical control mechanics. Simple gameplay and 2D environments means a lot of retro games are simple to physically operate. This makes them accessible to a larger audience, makes them easier to pick up and play for short sessions, and there isn't a big re-learning curve if you don't have time to play for a month or twelve.
A few years ago, I remember playing one of the Assassin's Creed games on PS4 and having to stop somewhere in the middle due to lack of time or a shift in my priorities. After several months, I tried to pick it back up and I had basically forgotten how to play. The game is still unfinished.